Lance, physics is inexorable. Air will always rise to the highest point in its container underwater, so you always have to get a dump to a place where it is where the air wants to go. It should not be necessary to go completely vertical to dump from your shoulder dump, though; if it is, the dump take off is too far forward on the shoulder of the BC. It should be sufficient, with normally constructed BCs, to roll a little bit to your right, and perhaps a little bit head-up.
It is very likely that the reason you feel you have to go very head-down to dump from your rear dump is that your normal posture isn't completely horizontal. If you are head-up, then getting a little head-down to move the gas to the rear dump is going to feel like a BIG change.